Daniel C. Donato,Joseph B. Fontaine,John L. Campbell
Dynamics of dead wood, a key component of forest structure, are not well described for mixed-severity fire regimes with widely varying fire intervals. A prominent form of such variation is when two stand-replacing fires occur in rapid succession, commonly termed an early-seral “reburn.” These events are thought to strongly influence dead wood abundance in a regenerating forest, but this hypothesis has scarcely been tested. We measured dead wood following two overlapping wildfires in conifer-dominated forests of the Klamath Mountains, Oregon (USA), to assess whether reburning (15-yr interval, with >90% vegetation mortality) resulted in lower dead wood abundance and altered character relative to once-burned stands, and how any differences may project through succession. Total dead wood mass (standing + down) following the reburn (169 ± 83 Mg/ha [95%CI]) was 45% lower than after a single fire (309 ± 87 Mg/ha). Lower levels in reburn stands were due to, in roughly equal parts, additional combustion and greater time for decay. Although a single fire in mature forest both consumed and created dead wood (by killing large live trees), a reburn only consumed dead wood (few large live trees to kill). Charred biomass (black carbon generation) was higher in reburned stands by a factor of 2 for logs and 8 for snags. Projecting these stands forward (notwithstanding future disturbances) suggests: (1) the near-halving of dead-wood mass in reburn stands will persist for ~50 yr until the recruitment of new material begins, and (2) the reburn signature on dead wood abundance will remain apparent for over a century. These findings demonstrate how a single stochastic variation in disturbance interval can impart lasting influence on dead-wood succession, reinforcing the notion that many temperate forests exist in a state of dead-wood disequilibrium governed by site-specific disturbance history. Accounting for such variation in disturbance impacts is crucial to better understanding forests with complex mixed-severity disturbance regimes and with increasing stochasticity under climatic change.
Among the most universally applied conceptual models in forest ecology are those describing the succession of dead wood over a forest sere (e.g., Harmon et al. 1986, Spies et al. 1988, Bormann and Likens 1994, Sturtevant et al. 1997, Pregitzer and Euskirchen 2004, Hall et al. 2006, Bradford et al. 2009, Kashian et al. 2013). Changes in dead wood over time influence several key ecosystem functions including nutrient cycling, wildlife habitat, fuel for wildfires, and carbon storage (Harmon et al. 1986). Dead wood models often describe a more or less “U-shaped” dynamic over successional time, with a pulse of dead wood from a stand-initiating disturbance, decay of this pool over time, and recruitment of new dead wood via mortality in the next stand. This parsimonious framework has proven relevant across a diversity of ecosystems characterized by infrequent stand-replacing disturbances. A key uncertainty, however, is the degree to which stochastic variations in disturbance frequency may cause deviations from expected trends in dead wood abundance.
Understanding the influence of disturbance variability is crucial not only in light of changing climate and disturbances (Turner 2010), but because our knowledge of forest disturbance regimes has evolved since the foundational dead-wood models were described. For example, the systems from which those models (Harmon et al. 1986, Spies et al. 1988) were derived, the temperate coniferous forests of the Pacific Northwest (PNW), were traditionally thought to develop along a linear trajectory in between infrequent, singular stand-replacing disturbances (e.g., Munger 1930, Franklin et al. 2002). However, it is becoming clear that portions of these forests, like many across the temperate zone (e.g., Collins et al. 2009, Marcoux et al. 2013), are influenced by more complex disturbance regimes that include intermediate, mixed-severity, or temporally clustered disturbances (Donato et al. 2009b, Halofsky et al. 2011, Perry et al. 2011, Tepley et al. 2013). A prominent example is when a young forest regenerating after a stand-replacing fire is burned by a second stand-replacing fire early in succession (i.e., early-seral “reburn;” cf. Thompson et al. 2007, Collins et al. 2009, Donato et al. 2009b, Holden et al. 2010, Miller et al. 2012, Teske et al. 2012). Fire risk, spread potential, and mortality susceptibility are all high early in succession in many forest types (Martin et al. 1974, McIver and Ottmar 2007, Odion et al. 2010, Thompson et al. 2011, Stephens et al. 2012), making reburns a potentially important disturbance factor. Indeed, one of the earliest dead-wood succession papers (Spies et al. 1988) hypothesized that reburns could be a key driver significantly reducing dead wood abundance far into the next successional cycle. To date, this hypothesis remains largely unexplored.
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